Thebeam of a ship is its width at its widest point. Themaximum beam (B_MAX) is the distance between planes passing through the outer sides of the ship,beam of the hull (B_H) only includes permanently fixed parts of thehull, andbeam at waterline (B_WL) is the maximum width where the hull intersects the surface of the water.^[1]

Generally speaking, the wider the beam of a ship (or boat), the moreinitial stability it has, at the expense ofsecondary stability in the event of acapsize, where more energy is required to right the vessel from its inverted position. A ship thatheels on herbeam ends has her deck beams nearly vertical.^[2]

Typical length-to-beam ratios (aspect ratios) for small sailboats are from 2:1 (dinghies to trailerable sailboats around 20 ft or 6 m) to 5:1 (racing sailboats over 30 ft or 10 m).

Large ships have widely varying beam ratios, some as large as 20:1.

Rowing shells designed for flatwater racing may have length to beam ratios as high as 30:1,^[3] while acoracle has a ratio of almost 1:1 – it is nearly circular.

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^{[further explanation needed]}

The beam of manymonohull vessels can be calculated using the following formula:

${\displaystyle Beam=LO{A}^{\frac{2}{3}}+1}$

Where LOA is Length OverAll and all lengths are in feet.

Some examples:

• For a standard 27 ft (8.2 m) yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27 ft monohulls is 10 ft (3.05 m).
• For aVolvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around 18 ft (5.5 m).
• For a 741 ft (226 m) long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around 82 ft (25 m), e.g.Seawaymax.

As catamarans have more than one hull, there is a different beam calculation for this kind of vessel.

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BOC stands for Beam On Centerline. This term in typically used in conjunction with LOA (Length overall). The ratio of LOA/BOC is used to estimate the stability of multihull vessels. The lower the ratio the greater the boat's stability.

The BOC for vessels is measured as follows:For a catamaran: the perpendicular distance from the centerline of onehull to the centerline of the other hull, measured at deck level.For a trimaran: the perpendicular distance between the centerline of the mainhull and the centerline of either ama, measured at deck level

Look upbeam (nautical) in Wiktionary, the free dictionary.

Other meanings of 'beam' in the nautical context are:

• Beam – a timber similar in use to a floor joist, which runs horizontally from one side of the hull to the other athwartships.
• Carlin – similar to a beam, except running in a fore and aft direction.
• Beam – the direction across the vessel, perpendicular to fore-and-aft; something lying in that direction is said to beabeam.

• Hayler, William B.; Keever, John M. (2003).American Merchant Seaman's Manual. Cornell Maritime Pr.ISBN 0-87033-549-9.
• Turpin, Edward A.; McEwen, William A. (1980).Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press.ISBN 0-87033-056-X.

• Nautical terminology
• Ship measurements

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